I'm currently learning Newton's Laws, and I came across this question:
"A man lightly walks across a roof so that he does not break it. However, his friend tosses him a tool, causing him to jump in the air, and break through the roof. Why does the roof hold his weight when he walks, but not when he jumps?"
My confusion stems from the fact that in both cases there is a force of gravity and normal force acting upon him when he is on the ground. The only difference is his acceleration. And yet, if there is acceleration, there must be unbalanced forces, but what are those unbalanced forces caused by? How does he break through the roof?
Best Answer
You have already correctly identified the forces. The unbalanced force is simply the normal force being larger than before.
The normal force must now, apart from holding back against his weight, also create the upwards acceleration. Thus it grows.
If the surface is not strong enough to exert this new larger normal force, then the surface will break.